1. Field of the Invention
This invention relates to heating devices and in particular to an induction heating coil for use in heating a frerromagnetic utensil or the like.
2. Description of the Background Art
One form of cooktop for use in domestic ranges and the like comprises a magnetic induction cooktop wherein a work coil subjacent the smooth cooktop upper surface acts as a transformer primary coil in inducing secondary currents in an overlying ferrous utensil, or the like. A high frequency electric current passing through the work coil at about 25 kHz., produces a rapidly changing magnetic field. The electrical power input to the circuit which includes the work coil is in the range of 1000 to 2000 watts.
A serious problem has arisen, however, in the induction heating devices of the prior art in that nonuniform heating of the utensil occurs causing undercooking of portions of the food in the utensil and overcooking of other portions.
A number of different induction heating coil arrangements have been developed in an attempt to solve this vexatious problem. Thus, illustratively, Carl G. Eisen, in U.S. Pat. No. 2,751,480, discloses an induction heating coil having coil turns spaced relatively close together where thick sections are to be heated, and relatively far apart where thinner sections are to be heated.
Ronald J. Cunningham discloses in U.S. Pat. No. 3,843,857, an induction heating coil which is unsymmetrically arranged so that the amount of heating of a utensil placed thereon may be varied depending on the location at which the utensil is placed.
A work coil for use in an induction cooking appliance is disclosed in U.S. Pat. No. 4,029,926 of Buddy J. Austin, wherein the turns of the work coil are uniformly spaced.
An induction heating coil construction for use in a cooking appliance is disclosed in U.S. Pat. No. 4,296,295 of Mitsuyuki Kiuchi, wherein the work coil is constructed with relatively widely spaced turns adjacent the open center portion of the coil and closely spaced turns on the outer portion of the coil in an attempt to provide a more uniform heating over the total area of the bottom of the cooking utensil. However, it is apparent that this proposed solution to the problem is inadequate and therefore unsatisfactory in that there is no compensation provided for the open center portion of the work coil and therefore there would be a relatively cold zone near and around the center of the cooking utensil.